Set the Server singleton in the static Bukkit class, allows Bukkit.getServer()
Remove 'clean' from gradle defaultTasks. Improves build times

See comparison https://gist.github.com/deathcap/b6d787435b560b281b03
Replace build system with Maven, remove Gradle - ~30% speed up

For some reason, Gradle is noticeably slower on my system as compared to
Maven, even with a nearly identical project definition. Maven is about
30% faster or so, 2.4 seconds or less compared to Gradle at 3.5 seconds
or there abouts. Only an approximately one second difference but it
feels a lot slower when building with Gradle. Used to be 2x slower but
sped it up by removing one of the unnecessary default tasks, still
uncomfortably slow for my tastes. See console logs for comparison at
https://gist.github.com/deathcap/b6d787435b560b281b03

Also, the Bukkit dependency builds with Maven too, so it is more
consistent if we also use it, instead of Gradle. Oh well.
Add org.bukkit.World stubs
Rename World to WorldImpl to avoid naming clash with interface

Technically, we could use fully-qualified class names to avoid
conflicting org.bukkit.World with deathcap.junket.World, but it gets
confusing and ambiguous; renamed the latter to WorldImpl instead.
Rename Server to ServerImpl
Return theWorld from getWorlds(), getWorld(String), and getWorld(UUID) APIs
Some progress on WorldImpl
Add org.bukkit.block.Block implementation stubs

Now using the nifty 'Copy Javadoc' feature
Start implementing BlockImpl
Remove javadoc from API

On second thought, it clutters up the implementation too much, and
creates the problem of keeping it up-to-date. Removed javadoc comments,
and replaced @deprecated when appropriate with @Deprecated annotations.
Add org.bukkit.entity.Player stubs

No javadoc copy this time
Wire up getPlayers() in World to Player; first pass of a few simple APIs
Inventory stubs
Add player inventory member variable to Player